The present invention relates to a dynamic pressure bearing-unit and a method for manufacturing the same. The bearing unit is suitable particularly for supporting spindles of spindle motors of information equipment, for example, magnetic disk units for such as HDD and FDD, optical disk units for such as CD-ROM and DVD-ROM, and magneto-optical disk units for such as MD and MO. The bearing unit is also suitable for supporting spindles of polygon scanner motors of laser beam printers (LBP).
The spindle motors of the various types of the aforementioned information equipment require highly accurate revolution as well as high speed, low cost, and low noise. One of the components responsible for the performance required is the bearing for supporting the spindle of the motor. These days, as a bearing of this type, dynamic pressure bearings with outstanding properties to meet the aforementioned requirements for performance have been studied or in practical use.
FIG. 10 shows an example of a spindle motor of this type. The motor has a configuration in which a shaft member 22 (comprising a shaft 22a and a thrust disk 22b which becomes a flange portion when attached to the shaft 22a) rotatably supported by means of a bearing unit 21 is rotatably driven by means of the magnetic force produced between a motor stator 4 fixed to a bearing member 27 and a motor rotor 5 mounted to the shaft member 22. The bearing unit 21 is provided with a radial bearing 30 for supporting the shaft member 22 in the radial direction and a thrust bearing 31 for supporting the thrust disk 22b in the direction of thrust. Both the radial bearing 30 and 31 are dynamic pressure bearings which have grooves (dynamic pressure grooves) for producing a dynamic pressure on the bearing surfaces. The dynamic pressure grooves of the radial bearing 30 are formed on the inner periphery of the bearing member 27. On the other hand, the dynamic pressure grooves of the thrust bearing 31 are formed on the both end surfaces of the thrust disk 22b fixed to the lower end of the shaft member 22. The bearing member 27 is provided, on the bottom portion thereof, with a step portion having the depth of the thickness of the thrust disk 22b plus the width of the thrust bearing clearance (t=approximately 5 to 20 xcexcm). A back metal 33 is incorporated into this step portion, thereby forming thrust bearing clearances Cs1, Cs2 having the aforementioned predetermined depth on the both axial sides of the thrust disk 22b (t=Cs1+Cs2).
The bearing unit 21 is assembled as follows. First, the thrust disk 22b and the back metal 33 are incorporated into the bearing member 27. After that, the shaft 22a with a radius smaller than the inner radius of the bearing member 27 by radial bearing clearance Cr is inserted into the inner diameter portion of the bearing member 27. Then, the distal end of the shaft 22a is press-fitted to the inner diameter portion of the thrust disk 22b. It may adopt another process that the back metal 33 is incorporated into the bearing body 27 after the shaft member 22 assembled the shaft and thrust disk 22b is inserted into the bearing body 27.
In the aforementioned bearing unit 21, inaccuracy of the perpendicularity between the shaft 22a and the thrust disk 22b would cause the thrust disk 22b to contact with the surfaces opposite thereto in the thrust bearing clearances Cs1, Cs2 and thus the bearing performance to deteriorate. Therefore, in the assembly process, it is necessary to press-fit the shaft 22a to the thrust disk 22b with accuracy, however, the press-fitting hardly provides the accuracy required (of the order of 2 xcexcm in perpendicularity). Moreover, since the shaft 22a and the thrust disk 22b have been already incorporated into the unit, measurement of the perpendicularity or a check of the accuracy would be usually difficult. Even if possible, the measurement or the check would require time-consuming work, resulting in an increase in assembly cost.
Furthermore, in the aforementioned bearing unit, the dynamic pressure grooves on the both end surfaces of the thrust disk 22b can be formed by means of presswork at low cost. However, the dynamic pressure grooves of the inner periphery of the bearing member 27 need to be formed by means of purpose-built high-accuracy equipment to meet individual shapes of the bearing member 27, thereby raising the manufacturing cost. In addition, this structure requires time-consuming work such as measurement of dimensions to improve the accuracy of the bearing clearances of the thrust bearing, causing a concern to rise about an increase in cost due to an increase in labor.
In view of the aforementioned problems, a main object of the present invention is to reduce the cost required for manufacturing a dynamic pressure bearing-unit as well as improve the accuracy thereof. More specifically, a first object is to provide a dynamic pressure bearing-unit which can provide improved accuracy between the shaft and the thrust disk (such as perpendicularity) at low cost. A second object is to facilitate setting the bearing clearances (the thrust bearing clearances) of the thrust bearing as well as to implement highly accurate thrust bearing clearances.
In order to achieve the aforementioned first object, in a dynamic pressure bearing-unit comprising a shaft member composed a shaft and a flange portion, a radial bearing portion for supporting the shaft member in a radial direction, and a thrust bearing portion for supporting the flange portion of the shaft member in the direction of thrust, wherein the radial bearing portion and the thrust bearing portion support the shaft member by dynamic pressure action out of contact, respectively, the dynamic pressure bearing-unit according to the present invention is provided with the shaft and the flange portion which are constructed in one piece.
Such integrated structure of the shaft member readily can ensure accuracy such as the perpendicularity between the shaft and the flange portion. Moreover, the perpendicularity can be measured before the shaft and the flange portion are incorporated into the bearing unit, thereby facilitating accuracy measurement and the check thereof.
The radial bearing of the dynamic pressure bearing-unit is configured in such a manner that a bearing member is arranged along the outer periphery of the shaft member, and a radial bearing clearance of the radial bearing portion is formed between the outer periphery of the shaft member and the bearing member facing thereto.
The thrust bearing portion is to have two thrust bearing clearances on both sides of the flange portion. In this case, one end surface of the flange portion and the bearing member facing thereto (for example, the end surface thereof) can form one of the thrust bearing clearances of the thrust bearing portion. Moreover, a thrust support portion can be provided opposite to the other end surface of the flange portion, thereby allowing the thrust support portion and the other end surface of the flange portion to form the other one of the thrust bearing clearances of the thrust bearing portion.
The dynamic pressure grooves of the thrust bearing are preferably formed on either one of the bearing member and the thrust support portion (which is determined depending on the direction of thrust load) or on the both.
One end of the bearing member is preferably sealed with a sealing member. A labyrinth seal is preferable as the sealing member.
According to the present invention described above, the integrated structure of the shaft member readily provides improved accuracy such as the perpendicularity between the shaft and the flange portion, facilitating the accuracy measurement and the check thereof. Therefore, it is made possible to provide an inexpensive high-accuracy bearing unit suitable for use in a spindle motor of information equipment.
Furthermore, the dynamic pressure grooves of the thrust bearing may be formed on either one of the bearing member and the thrust support portion or on the both. In this case, even when the shaft member is made of a hard material such as an iron-based material, the bearing member or the thrust support portion may be made of soft metal, sintered metal or the like, which facilitates forming the dynamic pressure grooves. Thus, this enables reducing the cost of forming the thrust bearing surface.
In order to achieve the aforementioned second object, the dynamic pressure bearing-unit according to the present invention comprises a housing, a bearing member fixed to the housing, a shaft member, and a radial bearing portion and a thrust bearing portion which make use of dynamic pressure action produced at the time of relative rotation between the shaft member and the bearing member to conduct non-contact support of the shaft member. The aforementioned housing has a straight inner periphery in the shape of a cylinder with a bottom, and bearing clearances of the thrust bearing portion can be appropriately set by controlling a position of the bearing member relative to the housing.
Such a cylindrical housing integrated with a bottom (a bag-shaped housing) with a separate bearing member accommodated in the housing allows dynamic pressure grooves to be formed on the bearing member (particularly on the inner periphery thereof) by means of plastic work. This enables achieving high accuracy and low cost. Moreover, the bearing member can be formed in a simple shape without a step portion, for example, in the cylindrical shape, thereby facilitating handling the bearing member at the time of forming dynamic pressure grooves. When the bearing is incorporated into the housing, it is made possible to obtain readily thrust bearing clearances having appropriate widths with high accuracy by controlling the position at which the bearing member is fixed to the housing and defining appropriately the relative position between the both (particularly, the relative position in the axial direction).
The bearing clearances of the thrust bearing portion can be formed by removing a positioning means for positioning the bearing member relative to the housing. Since the width of the clearances corresponds to the size of the positioning means (particularly, the axial size), appropriate bearing clearances (thrust bearing clearances) of an appropriate width can be formed by changing the shape, the size, or the like of the positioning means.
The aforementioned dynamic pressure bearing-unit is manufactured by accommodating the shaft member and the bearing member in the cylindrical housing with a bottom to form the aforementioned radial bearing and thrust bearing. At this time, the bearing clearances of the thrust bearing can be set to appropriate widths by controlling the position at which the bearing member is fixed to the housing. In this case, the thrust bearing clearances of a desired width can be obtained only by adjusting the position at which the bearing member is fixed to the housing, thereby enabling reducing the manufacturing cost and providing the bearing clearances with improved accuracy.
The position at which the bearing member is fixed to the housing can be controlled by means of a positioning means provided inside the housing for positioning the bearing member relative to the housing. The thrust bearing clearances of an appropriate width can be obtained easily by removing the positioning means after positioning has been completed.
For example, the positioning means may be formed of a resin layer. In this case, a method for removing the resin layer may be achieved by means of a solvent. In the case where the bearing member is positioned relative to the housing, particularly, in the axial direction by means of the resin layer, the bearing member is fixed in the housing to a position added by the thickness of the resin layer. Therefore, the subsequent removal of the resin layer with a solvent allows for forming a clearance (an axial clearance) of a width corresponding to the thickness of the resin layer between the bearing member and the housing. Thus, it is made possible to make use of the clearance as the bearing clearances of the thrust bearing.
Specifically, the aforementioned bearing member can be positioned, for example, with the shaft member and the resin layer being interposed between the bearing member and the bottom of the housing, and with the bearing member, the shaft member, the resin layer, and the bottom of the housing being in intimate contact with one another.
The resin layer may be formed in the shape of a sheet or in the shape of a film.
According to the present invention described above, the cost of manufacturing a dynamic pressure bearing-unit can be reduced and dynamic pressure grooves can be formed with high accuracy. In particular, since the thrust bearing clearances can be formed in a simple process with accuracy, it is made possible to seek for further cost reduction as well as improve the stability and reliability of operation.
These and other objects and advantages of the present invention will become clear from the following description with reference to the accompanying drawings.